In an image forming apparatus, a semiconductive roller produced, for example, by forming a semiconductive rubber composition into a tubular body, crosslinking the tubular body and inserting a shaft such as of a metal into a center through-hole of the tubular body is generally used as a charging roller for uniformly electrically charging a surface of a photoreceptor body, as a developing roller for developing an electrostatic latent image formed by light-exposing the electrically charged photoreceptor surface into a toner image, as a transfer roller for transferring the formed toner image onto a paper sheet or the like, or as a cleaning roller for removing toner from the photoreceptor surface after the transfer of the toner image onto the paper sheet or the like.
In general, the semiconductive rubber composition to be used as a material for the semiconductive roller is imparted with ion conductivity by using an ion-conductive rubber (e.g., an epichlorohydrin rubber) as a rubber component.
Further, a diene rubber is generally used in combination with the ion-conductive rubber as the rubber component in order to improve the mechanical strength and the durability of the semiconductive roller or to improve the rubber characteristic properties of the semiconductive roller, i.e., to make the semiconductive roller more flexible and less susceptible to permanent compressive deformation with a reduced compression set.
Further, an outer peripheral surface of the semiconductive roller is generally covered with a coating film such as of a urethane resin.
The coverage of the outer peripheral surface of the semiconductive roller with the coating film is advantageous for the following reason. When the semiconductive roller is used as the charging roller or the like in direct contact with the photoreceptor body, the image formation is prevented from being influenced by contamination of the photoreceptor body with substances bleeding or blooming on the outer peripheral surface from the inside of the semiconductive roller.
Further, minute particles (external additives) such as of silica and titanium oxide externally added to the toner for improvement of the fluidity and the electrical chargeability of the toner, or broken toner particles (which are hereinafter collectively referred to as “external additives”) are prevented from adhering to the outer peripheral surface of the semiconductive roller and gradually accumulating on the outer peripheral surface to influence the image formation.
The coating film is generally formed by applying a coating liquid onto the outer peripheral surface of the semiconductive roller by a coating process such as a spraying method or a dipping method, and then drying the coating liquid. Therefore, the coating film is liable to suffer from contamination with dust and other foreign matter, uneven thickness and other defects during the coating process. Particularly, if the semiconductive roller suffering from any of these defects is used as the charging roller, it is impossible to uniformly electrically charge the surface of the photoreceptor body. Problematically, this may result in defective image formation such as uneven image density.
In addition, the coating film formation technique, which is an established technique, has little room for improvement. Therefore, it is difficult to significantly reduce the incidence of the defects (defect percentage) as compared with the current technique. This may reduce the yield and the productivity of the semiconductive roller to increase the production costs.
In compact and less expensive laser printers and the like for use in small offices and for personal use, the coating film is entirely obviated for cost reduction, or a thin oxide film is formed instead of the coating film (see, for example, Patent Document 1).
Where the semiconductive roller is formed from the semiconductive rubber composition containing the diene rubber as the rubber component, the oxide film is formed in the outer peripheral surface of the semiconductive roller by irradiating the outer peripheral surface with energy radiation such as ultraviolet radiation or electron radiation to oxidize the diene rubber in the outer peripheral surface.
This eliminates the possibility that the oxide film is contaminated with dust and other foreign matter during the formation of the oxide film. Further, the oxidation reaction uniformly proceeds in the outer peripheral surface of the semiconductive roller, so that the oxide film is free from variations in thickness. Particularly, when the semiconductive roller is used as the charging roller, the surface of the photoreceptor body can be uniformly electrically charged, thereby advantageously preventing the defective image formation such as image density unevenness.
However, the crosslinked rubber has higher friction, and is highly adhesive. Where the coating film is obviated or the thin oxide film is formed instead of the coating film, the external additives are liable to adhere to the outer peripheral surface of the semiconductive roller to gradually accumulate on the outer peripheral surface. This problem is particularly remarkable in the case of the charging roller which is constantly kept in contact with the surface of the photoreceptor body.
The accumulating external additives may influence the characteristic properties of the semiconductive roller, e.g., the capability of the charging roller for charging the photoreceptor body, or may adhere again on an image formed on a paper sheet to cause defective image formation.
In recent years, image forming apparatuses are required to form higher-definition images at a higher process speed (at a higher image formation speed), and constituent components of the image forming apparatuses are required to be more durable. Therefore, the charging roller and the like are required to be substantially free from reduction in performance due to the adhesion and the accumulation of the external additives and capable of suppressing the defective image formation for a longer period of time.